Annotate of /cvsgraph/README

Fix a bug in the detection of branch-numbers in the logical ,v file structure
(reported by Daniel Dumitrache <ddumitrache {at} bitdefender.com>). The branch was
deduced from the revision, but that would fail for revision numbers that are
like magic branch-numbers (x.x.0.x) because they were interpreted. These
revision numbers are now correctly handled by treating them as revision numbers
and deducing the branch by eliminating the trailing .x.